Creating multichannel audio material from either standard matrix encoded two-channel stereophonic material (in which the channels are often designated “Lt” and “Rt”) or non-matrix encoded two-channel stereophonic material (in which the channels are often designated “Lo” and “Ro”) is enhanced by the derivation of surround channels. However, the role of the surround channels for each signal type (matrix and non-matrix encoded material) is quite different. For non-matrix encoded material, using the surround channels to emphasize the ambience of the original material often produces audibly-pleasing results. However, for matrix-encoded material it is desirable to recreate or approximate the original surround channels' panned sound images. Furthermore, it is desirable to provide an arrangement that automatically processes the surround channels in the most appropriate way, regardless of the input type (either non-matrix or matrix encoded), without the need for the listener to select a decoding mode.
Currently there are many techniques for upmixing two channels to multiple channels. Such techniques range from simple fixed or passive matrix decoders to active matrix decoders as well as ambience extraction techniques for surround channel derivation. More recently, frequency domain ambience extraction techniques for deriving the surround channels (see, for example, reference 1) have shown promise for creating enjoyable multichannel experiences. However, such techniques do not re-render surround channel images from matrix encoded (LtRt) material because they are primarily designed for non-matrix encoded (LoRo) material. Alternatively, passive and active matrix decoders do a reasonably good job of isolating surround-panned images for matrix-encoded material. However, ambience extraction techniques provide better performance for non-matrix encoded material than does matrix decoding.
With the current generation of upmixers the listener is often required to switch the upmixing system to select the one that best matches the input audio material. It is therefore an object of the present invention to create surround channel signals that are audibly pleasing for both matrix and non-matrix encoded material without any requirement for a user to switch between decoding modes of operation.